T cell receptors are heterodimeric immunoglobulin proteins, with approximately 30,000 on the surface of each clonally unique T cell (Janeway, Immunobiology: the Immune System in Health and Disease. 6th ed. New York: Garland Science; 2005). The specific binding of αβ TCRs to peptides bound by MHC proteins (pMHC) is a critical step in the antigen-specific T cell-mediated immune response. As with antibodies, the antigen-binding Complementarity Determining Region (CDR) loops are variable, with multiple germ-line segments, junctional combinations, and addition or deletion of nucleotides at these junctions used to generate diversity.